BALANCE OF PRESSURES
FORMATION PRESSUREOnce the well is shut-in providing nothing has broken down, the pressures in the well will be in balance. What is lacking in hydrostatic head of fluid in the well is now being made up by surface applied pressure on the annulus and on the drill pipe.
Providing the bit is on bottom and the string is full with a known mud density this allows us to determine what the formation pressure is and hence what kill mud weight is required to achieve balance.
On the drill pipe side of the U-tube.
On the casing side of the U-tube
The mixture of mud and formation fluid in the annulus makes it impossible to determine formation pressure using the casing information. The drill pipe, however, is full of clean mud of known weight and can be used as a barometer of what is happening downhole.
KILL MUD WEIGHT
We require the mud to produce a hydrostatic pressure equal to the formation pressure over a length equal to the true vertical depth of the hole. This can be expressed as a gradient, and converted to any desired mud weight unit; in this case ppg.
The kill mud weight required could also be described as the original mud weight increased by an amount which will provide a hydrostatic pressure equal to the amount of the drill pipe shut-in pressure over the vertical length of the hole.
Once the formation pressure is known, the mud weight required to balance, or kill, it can be calculated, since:-